1. Technical Field
The present invention generally relates to a method for determining chlorine demand in water.
2. Description of the Related Art
In order to insure that the water in a pool or spa is safe, it must be properly sanitized to prevent any health problems arising due to such contaminants as, for example, algae, bacteria, or any other pathogens which may be in the water. Thus, it is the goal of any owner or operator of recreational water bodies, swimming pools, spas, hot tubs or the like to provide pool water which is maintained so that there are no detrimental contaminants. To this end, the pool owner or operator may choose from a wide variety of biocidal chemical systems to ensure that a biocidally effective amount of water-treating agents is present in the water body on a continuous basis.
The more commonly used biocidal agents are chlorine-containing biocides. The chlorine can be in a number of different forms, e.g., sodium hypochlorite (liquid bleach), calcium hypochlorite, lithium hypochlorite, chlorinated isocyanurates, etc. When any of these materials interact with water, they undergo hydrolysis to form free chlorine consisting of predominantly hypochlorous acid (HOCl), which is the sanitizing agent, and hypochlorite ion.
In order to determine whether the amount of chlorine present is adequate to effect disinfection, measurements are made beyond the chlorine input point. The amount of chlorine added to the water is ordinarily referred to as the “dosage,” and is usually expressed as parts per million (ppm). The amount of chlorine used up or consumed by the contaminants, e.g., bacteria, algae, organic compounds and some inorganic substances, such as iron or manganese, is designated as the “demand.” Since many of the reactions with chlorine are not instantaneous, but require time to reach completion, chlorine demand is time-dependent.
The amount of chlorine remaining in the water at the time of measurement is referred to as the “residual chlorine.” Residual is therefore determined by the dosage subtracted from the demand. Inasmuch as chlorine demand is time-dependent, this dependency is likewise true of chlorine residual.
When chlorine dissolves in water, a mixture of hypochlorous and hydrochloric acids is formed. Actually, the hydrochloric acid always completely dissociates into hydrogen and chloride ions, whereas the hypochlorous acid only partially dissociates into hydrogen and hypochlorite ions. In either the hypochlorous acid or hypochlorite ion form, chlorine is called “free chlorine residual.” Free chlorine residual has a highly effective killing power toward bacteria.
The National Spa and Pool Institute recommends 1 to 3 parts per million of residual chlorine in water and a pH between 7.2 and 7.8. Presently, chlorine demand testing in pool water samples has been performed by preparing a stock solution of chlorine, dosing a control sample of distilled water and a test pool water sample and then allowing the samples to incubate. However, this method is relatively slow as it ordinarily takes eight to twenty-four hours for the samples to incubate.
Accordingly, it would be desirable to provide an improved method for determining chlorine demand in water, e.g., pool and spa water, such that a suitable amount of chlorine is present in the water to ensure, for example, proper sanitation of the water.